Is Mutt a dog? That's what a sources close to legendary music producer Robert "Mutt" Lange and estranged Shania Twain are saying.

Lange, who has been married to Twain for 14 years, reportedly strayed, but he didn't go far: He's been carrying on an affair with the woman who manages their Swiss chateau and serves as his secretary, Marie Ann Thiebaud, sources tell People.

Twain and Lange, who has produced albums with Def Leppard, AC/DC, and Celine Dion, revealed they were splitting last week. When reached by People, Lange attributed it to "just a growing apart, that's all," and denied an affair. But one source tells People that his secretary also left her husband, and that she and Lange are still in relationship.